The overall objective of the research is to define the association between subclinical hypothyroidism and prolactin activity and human breast cancer. Attention is being paid to the role which these factors may have in the etiology of the disease, and on the prognosis. In addition, a study is being made of the effect of adjuvant chemotherapy on pituitary-thyroid function in breast cancer. Plasma thyroid-stimulating hormone (TSH), thyroxin and prolactin levels are being assayed in three clinical situations (1) in apparently healthy women who are taking part in a breast cancer screening program (2) in patients awaiting mastectomy or about to commence adjuvant chemotherapy, and at 6 monthly intervals during adjuvant therapy, (3) in patients with colorectal cancer immediately prior to adjuvant chemotherapy or at the time of recurrence without prior treatment. In the cancer patients, plasma TSH and prolactin levels are determined before and after thyrotropin-releasing hormone (TRH) stimulation. In addition, plasma growth hormone and FSH concentrations are determined on these samples. BIBLIOGRAPHIC REFERENCES: Rose, D.P., Davis, T.E., and Fern M. Plasma TSH and prolactin responses to TRH in early breast cancer. Clin. Res., 24, 564 A, 1976.